In long term evolution advanced (LTE-A), the concept of a Relay Node (RN) has been introduced. The RN is a node deployed between an eNodeB (eNB) (i.e., base station) and a wireless transmit/receive unit (WTRU). The RN receives data from one of the eNB and the WTRU and forwards it to the other. The WTRU link quality to and from the RN would be better than the direct link to and from the eNB because the RN is closer to the WTRU, and therefore the performance of the link between the eNB and the WTRU would be increased by introducing the RN.
The RN is intended to be a low cost option to installing an eNB when an improved coverage is needed but it may also be used to improve the capacity of the radio network. The cost reduction is achieved by eliminating the capital and operating expenses associated with a wired link to the network. Instead of a wired link, the RN communicates wirelessly to a “donor eNB” (DeNB) which provides a link to the network. To legacy WTRUs, (i.e., third generation partnership project (3GPP) release 9 or earlier WTRUs), the RN looks just like an eNB. FIG. 1 shows an example deployment of the RN along with the DeNB and also shows the interfaces related to the RN and the DeNB.
With RN deployment in LTE-A, a handover between the RNs and the eNBs may occur. The following handover scenarios are expected: a WTRU moving from an RN to its own serving donor eNodeB, a WTRU moving from an RN to a neighboring donor eNodeB, a WTRU moving from one RN to another RN served by the same donor eNodeB, a WTRU moving from one RN to another RN served by a neighboring donor eNodeB, a WTRU moving from a donor eNodeB to an RN served by the same donor eNodeB, a WTRU moving from a neighboring donor eNodeB to an RN served by a different donor eNodeB, etc.
When the WTRU needs to be handed over from an RN (either to another RN or to a DeNB), in the case of radio link control (RLC) unacknowledged mode (UM), the packet data convergence protocol (PDCP) service data units (SDUs) in the RN for which the transmission in the downlink has not yet been completed by the RLC should be forwarded to the DeNB via the wireless backhaul link. In the case of RLC acknowledged mode (AM), for downlink transmission, in addition to the PDCP SDUs that could not be transmitted, the RN also has to forward the PDCP SDUs that have not been acknowledged successfully at the RLC layer by the WTRU. These data packet re-forwarding instances are necessary because the DeNB may have deleted the PDCP SDUs transmitted earlier to the RN, given the fact that both the backhaul link (Un) and the access link (Uu) operate independently and implement separate peer-to-peer data link level transmission protocol stacks, as shown in FIGS. 2-9. Data re-forwarding wastes backhaul resources twice, (i.e., the resources used to transmit the data from the DeNB to the RN, and the resources used to retransmit the data from the RN to the DeNB).
Examples of user plane (U-plane) and control plane (C-plane) protocol architecture as being considered in LTE-A are shown in FIGS. 2-9. FIGS. 2 and 3 show C-plane and U-plane architectures for full-L3 relay, which is transparent for the DeNB. FIGS. 4 and 5 show C-plane and U-plane architectures for proxy S1/X2, (i.e., the RN looks like a cell under the DeNB to the mobility management entity (MME)). FIGS. 6 and 7 show C-plane and U-plane architectures where the RN bearers terminate in the RN. FIGS. 8 and 9 show C-plane and U-plane architectures where the S1 interface terminates in the DeNB.
It would be desirable to reduce or avoid data re-forwarding from the RN to the DeNB during handover, and to minimize the amount of data transferred from the DeNB to the RN that needs to be re-forwarded back to the serving DeNB during handover, and reduce the handover completion time due to unnecessary data exchange between the RN and the serving DeNB.